SYMMETRICAL CURRENT BASIS OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

It has become a widely adopted practice
to determine the interrupting capability of circuit breakers in
kiloamperes symmetrical. The rated short-circuit current in rms
kiloamperes is referred to the rated maximum voltage in kilovolts.

The ratings structure and tables of
ratings for ac high-voltage circuit breakers are found in IEEE
C37.042 and IEEE C37.063.

The short-circuit current interrupting
process is characterized first by an arc appearing between the
breaker contacts. The arc contains a high conductivity plasma column
originating from the high temperature and related gas ionization (in
the case of gas-blast interrupters).

Interruption will occur at current zero
and in this case is first determined by successful cooling of the arc
(through gas flow) to eliminate the ionized gas conductive path, and
then the race to build up dielectric strength of the open contact gap
faster than the rise of the power system recovery voltage.

Several specific problems are
encountered during the interrupting process of gas-blast
interrupters:

1. Arc plasma temperatures exceeding
20,000 K.

2. The turbulent supersonic flow of the
quenching gas in a changing flow geometry with speeds ranging from a
few hundred meters per second to several thousand meters per second.

3. The interrupter-moving system and
its drive accelerates the moving masses in the few thousandths\ of a
second to speeds as high as 10 m/s while simultaneously compressing
the quenching gas.

4. The stress places on the network
system by the current interruption and the recovery voltage.

The interrupting principle of an SF6
puffer-type interrupter is sketched in Fig. 10-60. On opening, the
fixed and moving contacts are pulled apart by the operating
mechanism. Thus, the fault current is forced to flow along the arc
plasma.

The contact movement combined with the
compression cylinder movement in the opposite direction compresses
the quenching gas inside the cylinder. The quenching gas is
consequently forced to flow through the contact system, and the
insulated nozzle toward the exhaust.

This intensive flow of quenching medium
along the arc rapidly removes the energy converted within the arc
plasma and transforms the path between the open contacts into an
insulating gap.